1. Field of the Invention
This invention relates to a printed circuit board, more particularly to the structure of a printed circuit board having through-holes and thermal lands.
2. Description of the Related Art
Electronic components formed with lead pins (such as transistor modules) are commonly solder-mounted on a printed circuit board by the insert-mount method. Specifically, what is called “flow-soldering” is commonly adopted in which the lead pins of the electronic components are first inserted into through-holes formed in the printed circuit board and soldering is then conducted by bringing the printed circuit board into contact with molten solder. Owing to environmental concerns, use of lead-free (Pb-free) solder has become the norm in recent years.
In flow-soldering, the soldering is achieved by causing the molten lead-free solder to rise into (fill) the gaps between the through-holes and the lead pins. In the course of the soldering, the heat of the molten lead-free solder escapes via the conductor patterns connected to the through-holes. This escape of the solder heat has the undesirable effect of causing the solder to fall below its solidification temperature and solidify before it fills the gaps between the through-holes and lead pins, i.e., before the solder rises to the prescribed level.
It is known that the escape of heat can be inhibited by forming thermal lands between the through-holes and the conductor patterns as taught by, for example, Japanese Laid-Open Patent Application No. Hei 9(1997)-8443 ('443), particularly paragraphs 0005 and 0030-0032, and FIGS. 4-6 and 9.
However, the heat capacity of a conductor pattern increases in proportion as its surface area increases. As a result, the amount of solder heat escaping through a conductor pattern connected to a thermal land increases when the conductor pattern is of relatively large area (e.g., when it is a “broad pattern” such as one used for power supply or grounding). The inconvenience of the solder solidifying before rising to the prescribed level therefore persists in such cases. This leads to the conclusion that mere formation of thermal lands between the through-holes and conductor patterns is not sufficient in the point of improving the solder rise property. In order to prevent solder heat from escaping to the conductor patterns it is possible, as shown in FIGS. 4 to 6 of the above reference '443, to reduce the number of linear regions of the thermal lands or to make their width narrower. However, this solution is difficult to implement owing to the current-carrying capacity required between the through-holes and conductor patterns.
In response to this difficulty there has emerged the teaching of Japanese Laid-Open Patent Application No. 2002-217533 ('533; particularly paragraphs 0016 and 0017, and FIG. 1), which is to bring bent lead pins into contact with the land regions of the through-holes so as to transfer the heat of the lead pins to the land regions, thereby heating the land regions and preventing the temperature of the molten solder from falling sharply.
However, the diameter of the printed circuit board through-holes is ordinarily made only slightly larger than the diameter of the lead pins. Therefore, when there are multiple through-holes and lead pins and the lead pins are all of substantially the same diameter, all of the through-holes are also formed to the same diameter. As a result, the gaps between the through-holes and the lead pins also all come to have about the same size. Specifically, the size of the gaps is typically made uniform at about 0.3 mm.
Therefore, when a configuration like that of the reference '533 is adopted that brings bent lead pins into contact with the land regions of the through-holes, the tiny openings at the through-holes (near the exit of the gaps between the through-holes and the lead pins) are partially blocked to decrease the areas thereof. This makes it hard for gases generated during soldering, specifically the air in the through-holes, vapor generated owing to evaporation of moisture contained in the printed circuit board, and the like, to discharge from the openings. A problem of the molten solder not rising to the prescribed level is therefore liable to occur.
In addition, the need to bend the lead pins inconveniently increases the number of lead forming processes (i.e., production processes).
Further, since no fillets are formed at the points where the land regions make contact with the bent lead pins, the amount of solder constituting the fillets decreases. As a result, the strength with respect to a relatively large mounted electronic component may be insufficient, in which case defective connections are liable to occur.